1. Field of the Invention
This invention relates to a communication link with two-way communication of data, and more particularly, to a compensation for interruption of data flow along the link during both long and short intervals of data interruption, thereby to reduce any resulting communication errors.
2. Brief Description of Related Developments
Communication systems are widely used in many situations including communication between persons, as in cellular telephony, and between various forms of equipment, such as between a satellite and a ground station. Various data formats and protocols have evolved to facilitate communication in differing situations. Communication may involve multiple access technologies such as CDMA (code division multiple access), TDMA (time division multiple access), FDMA (frequency division multiple access), modulation technologies such as PSK (phase shift keying), QAM (quadrature amplitude modulation), and FEC (forward error correction) such as Reed Solomon coding, convolutional encoding, and turbo coding, by way of example. Detection of such signals may require a highly accurate time base for observation of relatively small differences in phase of a carrier signal, phase of the symbol and phase of the multiplexer frame. Furthermore, the time base employed in a receiver of a signal must be the same as the time base (synchronous) employed in a transmitter of the signal in order to enable successful operation of receiving processes (demodulation operation), such as matched filtering, by way of example. In the case of a communication system employing a modem at each end of a communication link, such as a link connecting two computers for enabling communication between the two computers, it is necessary to include within each of the modems circuitry for transmitting synchronization or time-frame signals and circuitry for recognizing received synchronization or time-frame signals.
Communication systems generally provide for continuous transmission of the data between a transmitting station and a receiving station. In the design of a typical communication system, such as a communication system carrying digitized data, it is presumed that there will not be any significant interruption in the flow of data, particularly an interruption that might arise from the temporary presence of a physical object which blocks the transmission path. Such an interruption, even a relatively short interruption on the order of a few milliseconds, could cause a loss in bit-count-integrity (BCI) so that data loss would continue even after termination of the interruption until such time as the transmitted signal could be reacquired by the receiving station.
By way of example in a communication system, one may consider first and second modems communicating with each other via a communication satellite which encircles the earth. The first modem is located on a ship at sea, and the second modem is located on land. To demonstrate the problematic situation addressed by the present invention, it is presumed that certain portions of the superstructure of the ship extend higher than communication antennas of the ship so that, depending on the orientation of the ship relative to one of its communication antennas and to the communication satellite, there are times when the superstructure of the ship may block the path of propagation of electromagnetic signals from a communication antenna to the satellite. To obviate most of this problem, two communication antennas may be employed in alternative fashion, whereby communication between the ship modem and the shore modem is directed either via the first or the second of the communication antennas. Thereby, in the event that the ship turns, such that there may be blockage of one of the antennas from view of the satellite, the other of the antennas is connected to the ship modem to resume the communication. Such a solution to the problem works well in that communication is always possible. However, a portion of the problem still remains, namely, that during the switching of the antennas there is an interval of time, on the order of milliseconds, in which data may be lost. By way of example, in the case of transmission from the shore modem via the satellite to the ship modem, the interval of lost data is approximately one millisecond. On the other hand, in the case of transmission from the ship modem via the satellite to the shore modem, it may be necessary to turn off the ship's transmitter for the first antenna and then to turn on the ship's transmitter for the second antenna with a resulting lost-data interval of approximately 80 milliseconds. The foregoing situation involving the two antennas is a specific example of the more general case of an interruption interval appearing in a handover between components of a communication system, such as a mobile station being handed over between two base stations in a cellular telephony system.